1. Field of the Invention
This invention relates to optical scanning and more particularly to such scanning effected by mirrors.
2. Discussion Related to the Problem
Optical scanning apparatus such as that used in a video player for scanning still pictures (e.g. photographic prints or slides) to produce a television signal, is well known. In a video player a picture is scanned in a horizontal direction at a standard television line rate, and the line scan is displaced in a vertical direction at the standard television field rate. In the NTSC standard television signal format, the picture is scanned from top to bottom 60 times per second. Approximately 1 millisecond is available between successive scans for retracing from the bottom of the picture back to the top. Typical prior art optical scanners employ a CRT to effect the horizontal and vertical scans. Achieving vertical retrace in less than 1 millisecond in such so-called "flying-spot" scanners is no problem.
Recent developments in solid-state line sensing arrays make them particularly attractive from cost, size and energy consumption standpoints, for use as an alternative to a CRT in a video player. A solid-state line sensing array can easily scan an image in the horizontal direction, and it remains to provide the required periodic vertical displacement of the horizontal scan with respect to the scanned image. The use of a pivoting mirror to periodically deflect an optical path in a scanner is well known in the art (see U.S. Pat. No. 2,590,281, issued Mar. 25, 1952 to G. C. Sziklai et al). In the standard NTSC television signal format, the vertical scan of the image takes about 16 milliseconds and as stated above, about 1 millisecond is allowed for retracing between vertical scans. To accomplish such a scan with a mirror, the mirror would be moved smoothly through some angular displacement in about 16 milliseconds then quickly returned to the initial starting position in about 1 millisecond. A plot of the mirror angular position versus time would resemble a "sawtooth" waveform. Unfortunately, because of the inertia of the mirror, practical pivoting mirror systems having inadequate high frequency performance to accomplish the vertical retrace in the approximately 1 millisecond time period available. One possible solution to this problem is to employ two pivoting mirrors, the angular displacement of each, versus time, resembling "back-to-back sawtooth" waveforms, which are 180.degree. out of phase. Scanning apparatus employing two mirrors shifts the optical path back and forth from one mirror to the other so that while one mirror is scanning a picture from top to bottom, the other mirror is pivoting back to its starting position (see U.S. Pat. No. 2,590,281 above). This arrangement greatly relaxes the high frequency response required of the mirror pivoting system, however, the complexity of two mirrors increases the expense of the apparatus and leads to added complication in aligning and matching the response characteristics of the two pivoting mirrors. Another way to accomplish quick retrace in a mirror scan system is to employ a rotating optical polygon as the deflecting element (see U.S. Pat. No. 4,148,071 issued Apr. 3, 1979 to Zinchuk). Unfortunately, optical polygons in themselves are relatively expensive optical elements.
The problem faced by the inventor, therefore, was to provide a scanning method and apparatus employing a pivoting mirror for deflecting an optical path, that (a) quickly retraced between successive vertical scans and (b) avoided the complexity and expense of prior art multiple mirror and rotating polygon solutions.